1. Field of the Invention
The present invention relates to an optical system adjusting method in an energy beam apparatus, and more particularly, the invention relates to an optical system adjusting method in an electron beam lithography apparatus that is used to form a fine pattern on a wafer.
2. Discussion of the Background
In the electron beam lithography used in recent semiconductor manufacturing processes, to increase the throughput, variable shaping or character projection type electron beam lithography apparatuses are used in which writing is performed by generating a beam having a cross-section of a rectangle, a triangle, or an arbitrary pattern in addition to the circular beam used in previous apparatuses. In this type of apparatus, because of the miniaturization and the increased accuracy of the pattern formation, the beam dimensions and position must be more accurate.
One condition to be satisfied for increasing accuracy in beam dimensions and position is that the optical axis of a beam and the axis of a lens should coincide with each other. For example, this condition is satisfied in a manner shown in FIG. 11, which shows an exemplary beam measurement using a conventional mark. A fine mark 112 is scanned with a beam 111, and a relative positional relationship between a scanning region and the mark 112 is determined based on a resulting reflected electron signal. Adjustments are made by using an alignment coil so that this positional relationship does not vary, even if the degree of lens magnetic excitation is changed.
The accuracy of an electron beam deflection position must also be very high. To correctly control the deflection, a mark position on a stage that is provided with a laser interferometer is determined under a certain deflection condition, and then the deflection condition is determined by moving the mark. A corresponding relationship between the deflection condition and the actual mark position is determined by repeating this operation, to thereby adjust a deflection system so that the, deflection position is located at a prescribed position.
A mark may be produced by forming a very small hole in a heavy metal thin film. The signal polarity is inverted at the mark. Further, an inflow current may be measured by using a minute Faraday cup structure as a mark. FIG. 12a shows another exemplary beam measurement using a conventional mark. FIGS. 12b-12c are graphs showing the signal intensity of reflected electron signals. A position is determined based on a signal as shown in FIG. 12b that is obtained by scanning a single mark 122 with a beam 121 as shown in FIG. 12a The middle of two peak positions may be obtained, for example, by differentiating a mark signal as shown in FIG. 12c. 
A signal obtained by scanning a mark with a beam typically includes considerable noise. To eliminate such noise, mark signals are obtained by scanning the mark with a beam many times and superimposing those signals one on another. In this case, it takes a long time to obtain sufficient accuracy. Further, applying a beam to the same location many times causes the mark and its vicinity to be heated locally. The resulting thermal expansion of the mark and the mark substrate may lower the accuracy.
As described above, the conventional method of adjusting the optical axis, the deflection position, the rotation, or the like of an energy beam by using a single mark has various problems, such as failure to obtain sufficiently high adjustment accuracy and the long time required to make adjustments.
The present invention has been made in view of the above circumstances in the art, and an object of the invention is therefore to provide an optical system adjusting method in an energy beam apparatus which can perform, correctly and in a short time, an optical system adjustment for optimizing the optical axis, the deflection position, or the rotation of an energy beam.
The present invention provides a method for adjusting an optical system of an energy beam apparatus, comprising preparing a mark having a one-dimensional or two-dimensional periodic structure; detecting a mark signal by scanning the mark with an energy beam one-dimensionally or two-dimensionally, and determining a variation in a positional relationship between the mark and a beam scanning region based on a phase variation of the mark signal.
In one aspect, a first mark signal is detected by scanning, with an energy beam, the mark that is set on the optical axis of the optical system. A second mark signal is detected by scanning, with an energy beam, the mark that is located at a position that is deviated from the optical axis of the optical system. A deviation of a deflection position is determined based on a phase difference between the first and second mark signals.
In another aspect, a first mark signal is detected by scanning, with an energy beam, a mark that is set on the optical axis of the optical system. A second mark signal is detected by scanning, with an energy beam, the mark in a state in which a driving condition of a lens that is being axially aligned is changed. A deviation between the energy beam optical axis and the axis of the lens is determined based on a phase difference between the first and second mark signals.
In another aspect, small fields that are smaller than energy beam deflection-test regions are set so that boundaries of adjacent ones of the small fields are in contact with each other. For adjacent small fields, mark signals are detected in an overlap region of deflection-test regions by using marks having the same periodic structure. The optical system is adjusted so that the two mark signals for the adjacent small fields coincide with each other.
In another aspect, a phase deviation of the mark signal is detected based on a phase deviation of a moirxc3xa9  signal that is obtained by calculating the product of the mark signal and a reference signal having a different frequency than the mark signal.
In another aspect, an offset-removed component of the mark signal is binarized. A phase deviation of the mark signal is detected based on a phase difference signal that is obtained by calculating the product of the binarized mark signal and a binarized reference signal having the same frequency as the mark signal and averaging a resulting product signal.
In another aspect, the mark has a two-dimensional periodic structure. A deviation in the rotational direction of a beam deflection region is detected based on a two-dimensional distribution that is obtained based on phase variations of periodic components of a mark signal distribution that correspond to the period(s) of the mark.
In another aspect, the mark has a two dimensional periodic structure. A second or higher order deviation of a beam deflection region from a designed deflection region is detected based on a two-dimensional distribution that is obtained based on phase variations of periodic components of a mark signal distribution that correspond to the periods of the mark.
In another aspect, a phase deviation of the mark signal based on a phase difference between moirxc3xa9 signals that are obtained for two reference signals that are higher and lower, respectively, in frequency than the mark signal.
In another aspect, a phase deviation of the mark signal is detected based on a phase difference signal that is obtained by calculating the product of the mark signal and a reference signal having the same frequency as the mark signal and averaging a resulting product signal.
In another aspect, the product of the mark signal and the reference signal is obtained by modulating the energy beam intensity at the frequency of the reference signal.
The energy beam may be an electron beam, an ion beam, a neutral particle beam, or a photon beam.
The present invention provides adjustment of, for example, the beam position on a sample correctly in a short time by determining a variation in the positional relationship between a mark and a beam scanning region based on a phase variation of a mark signal, thereby contributing to, for example, an increase of the rate of operation of an energy beam apparatus.